RFID technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer circuitry. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without requiring manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
Several advantages of RFID technology are summarized in Table 1:
TABLE 1Identification without visual contactAble to read/writeAble to store information in tagInformation can be renewed anytimeUnique item identificationCan withstand harsh environmentReusableHigh Flexibility/Value
As shown in FIG. 1, a basic RFID system 100 includes a tag 102, a reader 104, and an optional server 106. The tag 102 includes an integrated circuit (IC) chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a back-scattering modulator to send data back to the reader; anti-collision protocol circuits; and at least enough EEPROM memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information can then be passed to a server 106 for processing. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
The system uses reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range ˜3 m)        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum ˜3 m range)        Security & privacy protection        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        ˜100 meter range        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        Up to 30,000 meter range        
In RFID systems where passive receivers (i.e., Class-1 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
Fundamentally, the way a class 3 tag works, the reader sends out a carrier signal, typically at 900 MHz, and to communicate with the tag, it amplitude modulates the carrier. In order for the tag to communicate with the reader, the reader stops modulating, and the tag changes its reflectivity in order to communicate with the reader. This is called backscatter. One problem is that the reader cannot send out a perfect tone. The tone warbles, and this is known as phase noise. This noise gets confused with the signals coming back from the tag.
Another problem is that much of the signal received by the reader is not from the tag. The tag signal is very small compared to all of the potential signals found in the vicinity of the reader. Further, some of the reader-emitted carrier signal is found in the input to the reader. For instance, metal objects in the room reflect back carrier signal. Compounding the problem is that the carrier signal reflected from unwanted objects is variable. For instance, moving objects such as a forklift will cause the signal to vary due to the way the inherent change in reflectivity from a moving object. The signal will also vary when the reader is mounted on a moving platform. Thus, merely tuning the reader to optimize the signal for a particular environment is not enough. The variable nature of the carrier signal must be dealt with in order to effectively extract the tag signal from the noise created by other objects in the environment.
Thus, it is widely recognized that removing or canceling out as much of the reader's own carrier signal as possible is the key to achieving maximum reader sensitivity and range in backscattering RFID systems. One technique was investigated for subtracting out both the reader carrier wave and its associated phase noise from the return signals that feed into the reader's input. However, this technique only helps reduce cross-coupling in the reader itself and fails to reduce the effect of the massive un-modulated backscatter coming from reflective objects in the field of the reader.